Monte Carlo simulations of ripple filters designed for proton and carbon ion beams in hadrontherapy with active scanning technique

Proton and carbon ion beams have a very sharp Bragg peak. For proton beams of energies smaller than 100 MeV, fitting with a gaussian the region of the maximum of the Bragg peak, the sigma along the beam direction is smaller than 1 mm, while for carbon ion beams, the sigma derived with the same technique is smaller than 1 mm for energies up to 360 MeV. In order to use low energy proton and carbon ion beams in hadrontherapy and to achieve an acceptable homogeneity of the spread out Bragg peak (SOBP) either the peak positions along the beam have to be quite close to each other or the longitudinal peak shape needs to be broaden at least few millimeters by means of a properly designed ripple filter. With a synchrotron accelerator in conjunction with active scanning techniques the use of a ripple filter is necessary to reduce the numbers of energy switches necessary to obtain a smooth SOBP, leading also to shorter overall irradiation times.

We studied the impact of the design of the ripple filter on the dose uniformity in the SOBP region by means of Monte Carlo simulations, implemented using the package Geant4. We simulated the beam delivery line supporting both proton and carbon ion beams using different energies of the beams. We compared the effect of different kind of ripple filters and their advantages.